pig-iron. Sometimes the pig-iron is brought directly from the blast-furnace while fluid, and sometimes it is remelted in cupolas. In the early plants in England and America the lining of the vessel which held the iron was of ordinary silicious rock and clay, and this is still the universal practice in America. In other countries it has been necessary to develop a modification of the process, the linings being made of basic material, whereby the chemistry of the operation is greatly changed.
The growth of the basic Bessemer practice made it necessary to have a distinguishing name for the old way, and it is therefore called the acid process, the word being used in a chemical sense rather difficult to explain to any one not versed in chemistry.
In the acid process, the air passing through the iron burns the silicon and carbon, while the heat caused by their combustion furnishes sufficient heat to not only sustain the bath in a liquid state, but to increase its temperature, and to oftentimes necessitate the addition of scrap or steam as a cooling agent.
This increase in temperature is due principally to the silicon, which is of great calorific power, while the burning of the carbon gives barely sufficient heat for the bath to hold its own. It is necessary, therefore, that the iron contain sufficient silicon to raise the temperature to the point where steel will remain perfectly fluid. In the old days when operations in a steel works were slow and converters were allowed to cool off between charges, it was necessary for the pig-iron to have about 2 per cent, of silicon to get sufficient heat, but with the rapid methods of to-day, it is found that 1 per cent, is enough.
When the silicon and carbon are all burned, a certain amount of manganese is added in order that the steel shall be tough while hot, and be able to stand the distortions it is subjected to in the rolling mills. If soft steel is wanted, this manganese is obtained by using a rich alloy called ferromanganese, containing 80 per cent, of manganese, while if rail steel is being made, the usual method is to make a liquid addition of spiegel iron—a pig-iron containing about 12 per cent, of manganese.
For every ten tons of steel about one ton of this spiegel will be added, and this at the same time gives enough manganese to make it roll well, and enough carbon to confer the necessary hardness. When the rich alloy is used to make soft steel, as before explained,